Silicone compositions have long been used as release coatings, which are useful in many applications whenever it is necessary to provide a surface or material which is relatively nonadherent to other materials which would normally adhere thereto. Silicone release compositions are widely used as coatings which release pressure sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, Mylar.RTM., and other such substrates are also useful to provide non-stick surfaces for food handling and industrial packaging applications. For example, when self-adhesive labels are mounted on paper backing, it is desirable that the paper backing be easily peeled away from the label when it is ready for use. At the same time, the adhesive quality of the label should not be reduced by its having been peeled away from the backing. The same principle applies to various adhesive tapes which are wound on spools: The tape must unroll easily and still maintain its adhesive quality. Silicone release compositions coated on the surface of the label mounting paper, or the nonadhesive side of the tape, will prevent, respectively, permanent adhesion of the label to the mounting paper, or adhesion of the tape to the nonadhesive side of the tape as it is rolled.
Silicone compositions previously developed as release coatings have had to be dispersed in solvent in order to control the coatings' viscosity and thereby aid their application to substrates. Use of solvents, however, entails the highly inefficient process step of evaporating the solvent, which requires large expenditures of energy and costly pollution controls.
Because of these drawbacks, there has been a growing need for solventless coating compositions which are easy to apply to substrates. Such solventless coating compositions are sometimes referred to as "100% solids" compositions. The absence of solvent lowers the amount of energy required to cure such compositions and eliminates the need for expensive pollution abatement equipment.
Previously developed solventless polysiloxane release coatings, however, still require high oven cure temperatures in the neighborhood of 300.degree. F. minimum and are more expensive to use than solvent-dispersed or emulsion compositions. The high cure temperature requirement limits the usefulness of such coatings because only high-density papers or high melting point plastics and the like can be used as substrates without charring, melting or otherwise degrading during cure.
The higher cost of using solventless silicone release compositions stems from the nature of the curing reaction: ##STR1## Linear vinyl-functional polysiloxane polymers, which are the main component in solventless silicone release compositions, undergo this hydrosilation addition reaction during cure. Effective migration-free cure is obtained when .tbd.Si--CH.dbd.CH.sub.2 content is no less than 3 or 4 mole percent of the polysiloxane polymer. Such vinyl substitution to set up the addition reaction shown at (I) is very expensive; and vinyl substitution paired with use of expensive platinum addition-cure catalysts is responsible for the high cost of using solventless grades.
U.S. Pat. No. 4,256,870 to Eckberg (incorporated herein by reference) discloses silicone release compositions which can be cured at temperatures as low as 200.degree. F.; however, to effect an acceptable cure at these reduced temperatures, a large amount of platinum catalyst is required. For example, as much as eight times the amount of platinum (e.g., 200 ppm instead of 25 ppm) might be necessary for low temperature cure compared to that required for high temperature cure. Obviously, the high platinum concentration adds to the cost of such compositions, and there is consequently considerable current interest in limiting the level of required platinum without sacrificing low temperature cure performance.
In commonly assigned copending U.S. application Ser. No. 416,576, filed Sept. 10, 1982, incorporated herein by reference, and now abandoned, unsaturated organic monomers such as .alpha.-olefins are employed as carrier diluents to assist in introducing high levels of vinyl MQ resins into solventless silicone release compositions. The reactive MQ resins act as controlled release additives, allowing adjustment of the release (i.e., the force in grams required to separate an adhesive from a substrate coated with a cured release coating) of the silicone release compositions.
It has now been discovered that a certain class of .alpha.-olefins can be added directly to solventless addition cure polysiloxane release coating compositions to yield compositions of equivalent curing characteristics and release properties, but with a significant reduction in the level of precious metal catalyst required and in the level of vinyl functionality on the polysiloxane base polymer chain required. The .alpha.-olefins are also uniquely compatible with the contemplated polysiloxane compositions in terms of solubility, cure compatibility, release compatibility, low volatility and low toxicity.